Chapter 2
Configuration and Installation
2-20
ni.com
Figure 2-6.
Floating AC-Coupled Signal Connection
Figure 2-7.
AC-Coupled Signal Connection with High Common-Mode Voltage
For AC-coupled signals, you should connect an external resistor from the
positive input channel to the signal reference. This is needed to provide the
DC path for the positive input bias current. Typical resistor values range
from 100 k
Ω
to 1 M
Ω
. This solution, although necessary in this case,
lowers the input impedance of the input channel amplifier and introduces
an additional offset voltage proportional to the input bias current and to the
resistor value used. The typical input bias current of the amplifier consists
of ±80 pA and a negligible offset drift current. When a 100 k
Ω
resistor is
used, this will result into ±8 µV of offset, which is insignificant in most
applications. However, if larger resistors are used, significant input offset
may result. To determine the maximum offset introduced by the biasing
resistor, use the following equation:
V
s
V
out
+
–
+
–
R
bias
+
Module
V
s
V
out
+
–
+
–
R
bias
V
cm
High
CMV
+
–
+
Module
V
ofsbias
I
bias
R
bias
×
=